1. Field of the Invention
This invention relates to an apparatus for forming a deposited film, for example, a deposited film of an amorphous material comprising silicon atoms as the main component (hereinafter abbreviated as a-Si), etc. on a substrate.
2. Description of the Prior Art
In the following, this type of technique is explained by referring to an embodiment employing a-Si for an electrophotographic photosensitive member.
Generally speaking, an electrophotographic photosensitive member is constituted by laminating various kinds of functional layers such as photoconductive layer, charge injection preventive layer, protective layer, etc. on a substrate having a desired shape such as cylinder, etc. Of such electrophotographic photosensitive members, the electrophotographic photosensitive member employing a-Si as the functional layer as mentioned above is attracting attention as a substitute for the electrophotographic photosensitive member employing an amorphous selenium of the prior art for the various advantages possessed by a-Si, for example, little change with lapse of time, no environmental contamination, high surface hardness, etc.
FIG. 2 is a schematic illustration of the structure of a typical example of such an electrophotographic photosensitive member employing a-Si.
In FIG. 2, 1' is a substrate which is usually made of an aluminum drum. 2' is a charge injection preventive layer which prevents injection of charges from the substrate 1' into the photoconductive layer 3' and laminated on the substrate 1'. For example, when charging is effected by corona charging of .sym. polarity, it is necessary to prevent injection of .crclbar. charges from the charge injection preventive layer 2' into the photoconductive layer 3'. Ordinarily, the charge injection preventive layer 2' is constituted of a-Si doped with elements having trivalent valance such as boron (B) (the layer is of P.sup.+ a-Si), in which a mobility of electrons is made smaller by said elements to effect injection prevention of charges.
On the charge injection preventive layer 2' is laminated a photoconductive layer 3', which is ordinarily constituted of a non-doped a-Si layer. On this photoconductive layer 3' is further laminated a protective layer 4' to constitute an electrophotographic photosensitive member. The protective layer 4' is provided for further improvement of durability, and this is ordinarily constituted of an a-Si with broad band gap containing carbon (C) atoms (a-SiC layer). Thus, the constitution of an electrophotographic photosensitive member is generally made a four layer constitution of Al/P.sup.+ a-Si/non-doped a-Si/a-SiC.
The respective functional layers of such a photosensitive member are generally prepared according to the plasma decomposition method utilizing flow discharging. However, the plasma decomposition method involves the problems as mentioned below, and such problems are particularly marked in the case of preparing a layer with a thick layer thickness such as non-doped a-Si layer, which requires a layer thickness of about 20 .mu.m or more.
That is to say, (1) the gases such as SiH.sub.4, Si.sub.2 H.sub.6, etc. used as the starting gas tend to spontaneously combust, and film preparation is accompanied with danger and disposal of waste gases; (2) while decomposition efficiency of the starting gas is required to be increased in order to increase the film deposition speed, the applied voltage must be made higher for this purpose, but increased applied voltage will inevitably result in increase in species of radicals or ions generated by decomposition of the starting gas, whereby those having deleterious effects on the film characteristics may also be increased; (3) for obtaining uniform film characteristics, discharging is required to be effected uniformly, but such control of discharging is difficult for a product with large area such as an electrophotographic photosensitive member.
Methods which have been proposed for preparing an electrophotographic photosensitive member having excellent film characteristics by cancelling such problems are the so called pyrolysis methods in which radicals are formed. Among these, the method for preparing non-doped a-Si layer photoconductive layer through utilization of the pyrolysis reaction: ##STR1## is a method excellent with respect to safety, because (CH.sub.3).sub.3 SiH and (CH.sub.3).sub.3 Si.sub.2 H.sub.3 are gases that do not spontaneously combust. It has also the advantages that film characteristics are easily controlled due to relatively limited species of radicals being generated in the pyrolysis method as compared to the plasma decomposition method, as well as high film deposition speed. It is also possible to prepare P.sup.+ a-Si layer by using in the pyrolysis reaction a gas for doping such as B.sub.2 H.sub.6, etc. The film characteristics of these functional layers have better quality than those obtained by the plasma decomposition method.
However, concerning a-SiC layer, it has been difficult to prepare this according to the pyrolysis method, since both SiH.sub.2 and CH.sub.2 cannot be formed into the layer.